1. Field of the Invention
The present invention relates to a distributed optical fiber sensor capable of measuring strain and/or temperature in longitudinal direction with high accuracy and high spatial resolution using an optical fiber as a sensor.
2. Description of the Related Art
A method based on a Brillouin scattering phenomenon occurring in an optical fiber is known as a technology for measuring strains and temperatures. In this method, the optical fiber is used as a medium for detecting strain and/or temperature in an environment where the optical fiber is placed.
The Brillouin scattering phenomenon is such a phenomenon that, when two light rays having different frequencies pass each other in an optical fiber, power transfers via acoustic phonons in the optical fiber from the light having a higher frequency to the one having a lower frequency. If νd denotes a frequency difference between two light waves passing each other, the transferring power is proportional to a Brillouin gain spectrum BSg(νd) approximately defined by Equation 1:BSg(νd)=1/(1+(2(νd−νb)/Δνb)2)  Equation 1where νb is Brillouin frequency shift and Δνb is called Brillouin gain line width (full width at half maximum), these being parameters characterizing the Brillouin gain spectrum BSg(νd).
The Brillouin frequency shift νb is given by Equation 2:νb=2nva/λ  Equation 2where n is refractive index of the optical fiber, va is sound velocity in the optical fiber and λ wavelength of light incident on the optical fiber.
Since the sound velocity va depends on the strain and temperature of the optical fiber, the strain and/or temperature can be measured by measuring the Brillouin frequency shift νb.
Accordingly, Brillouin gain spectra at the respective parts of the optical fiber may be measured in order to measure a strain distribution and/or a temperature distribution of the optical fiber in the longitudinal direction of the optical fiber. In order to attain a high spatial resolution, the lengths of these parts need to be shortened.
FIG. 24 are diagrams showing the construction of a distributed optical fiber sensor and a probe light according to a background art, wherein FIG. 24A is a block diagram showing the construction of the distributed optical fiber sensor according to the background art, FIG. 24B is a chart showing first probe light and FIG. 24C is a chart showing second probe light. FIG. 25 is a chart showing Brillouin loss/gain spectra, wherein horizontal axis represents frequency and vertical axis represents loss/gain.
In FIG. 24, the distributed optical fiber sensor 500 according to the background art is provided with a probe light source 501, an optical coupler 502, a sensing optical fiber 503, a pump light source 504 and a detector 505.
The probe light source 501 generates an optical pulse shown in FIG. 24B and emits the generated optical pulse as probe light. The probe light emitted from the probe light source 501 is incident on one end of the sensing optical fiber 503 via the optical coupler 502. The sensing optical fiber 503 is an optical fiber for detecting strain and/or temperature in an environment where this optical fiber is placed and used as a sensor. The pump light source 504 generates continuous light having a frequency lower than that of the probe light and emits the generated continuous light (CW light) as pump light. The pump light emitted from the pump light source 504 is incident on the other end of the sensing optical fiber 503. In the sensing optical fiber 503, the probe light and the pump light cause a Brillouin scattering phenomenon, and light attributed to this Brillouin scattering phenomenon is incident on the detector 505 via the optical coupler 502. The distributed optical fiber sensor 500 measures the intensity of the light attributed to the Brillouin scattering phenomenon frequency by frequency in a time domain while successively changing the frequency of the pump light or the probe light, determines Brillouin gain spectra BSg(νd) in the respective parts along the longitudinal direction of the sensing optical fiber 503, and determines a strain distribution and/or temperature distribution along the sensing optical fiber 503.
Although the method for determining the strain and/or temperature from the Brillouin gain spectrum BSg(νd) is described above, strain and/or temperature can be similarly determined using a Brillouin loss spectrum BSl(νd) instead of the Brillouin gain spectrum BSg(νd) by setting the frequency of the pump light to be higher than that of the probe light.
The spatial resolution of this distributed optical fiber sensor 500 is restricted by the width of the optical pulse used for the measurement. Specifically, if vg[m/s] denotes velocity of the light in the optical fiber, spatial resolution Δz is vgTp/2 [m] in a measurement using an optical pulse whose width is Tp[s]. More specifically, in normally used ordinary optical fibers in which the velocity of light slightly differs depending on the materials of the optical fibers, a Brillouin gain spectrum BSg(νd) or a Brillouin loss spectrum BLg(νd) (hereinafter, abbreviated as a “Brillouin loss/gain spectrum BSl/g(νd)) is represented by a Lorenz (Lorentzain) curve (curve “a” shown in FIG. 25) up to the optical pulse width of 30 ns. If the optical pulse width is shortened to be below 30 ns, this results in a broadband curve (curve “b” shown in FIG. 25) to lose a peak in the vicinity of a mean frequency, thereby taking a moderate shape. Thus, the spatial resolution Δz becomes about 2 to 3 m. Although the optical pulse having a short width is necessary to improve the spatial resolution, the spectral width of the optical pulse becomes wider in this case, with the result that strain measurement accuracy becomes poor. Therefore, it has been difficult to measure strain and/or temperature distributions with high spatial resolution (of, e.g. 1 m or less) with high accuracy (of, e.g. 200μ∈), and there has been a demand to measure strain and/or temperature distributions with high accuracy and high spatial resolution.
Accordingly, as disclosed, for example, in the following documents 1 to 3, the probe light source 501 causes an optical pulse having a specified light intensity As2 to be incident on the sensing optical fiber 503 while causing continuous light (leakage light) having a weak light intensity Cs2 to be incident thereon as shown in FIG. 24C, whereby the Brillouin loss/gain spectra BSl/g(νd) substantially approximate to Lorenz curves having very steep peaks substantially at mean frequencies so that the mean frequencies can be clearly recognized as shown by the curve “a” in FIG. 25. In this way, it is known to measure strain and/or temperature with high accuracy and high spatial resolution.
Here, the Lorenz curve is generally expressed by Equation 3 that is a Lorenz function g(x):g(x)=1/πa/(a2+(x−a)2)  Equation 3Document 1
X. Bao and A. Brown, M. DeMerchant, J. Smith, “Characterization of the Brillouin-loss spectrum of single mode fibers by use of very short (<10-ns) pulses”, OPTICS LETTERS, Vol. 24, No. 8, Apr. 15, 1999
Document 2
V. Lecoeuche, D. J. Webb, C. N. Pannell and D. A. Jackson, “Transient response in High-resolution Brillouin-based distributed sensing using probe pulses shorter than the acoustic relaxation time”, OPTICS LETTERS, Vol. 25, No. 3, Feb. 1, 2000.
Document 3
Shahraam Afshar V., Graham A. Ferrier, Xiaoyi Bao, and Liang Chen, “Effect of the finite extinction ratio of an electro-optic modulator on the performance of distributed probe-pump Brillouin sensor systems”, OPTICS LETTERS, Vol. 28, No. 16, Aug. 15, 2003.
Since the setting of the light intensity Cs2 of the leakage light depends on the length of the sensing optical fiber, it has been necessary to manually make a fine adjustment in conformity with the length of the sensing optical fiber every time a measurement is conducted. Thus, if the distributed optical fiber sensor is an industrial product, a user needs to make this difficult manual adjustment, which has hindered the productization of distributed optical fiber sensors as industrial products.
In document 2, authors confirmed the phenomenon disclosed in document 1 by means of simulation, but shed no light in terms of theoretical analysis. Document 2 shed no light on the cause of being able to measure strain and/or temperature with high accuracy and high spatial resolution by causing an optical pulse having a specified light intensity to be incident on the sensing optical fiber while causing continuous light (leakage light) having a weak light intensity to be incident thereon. Thus, it was not clear how physical quantities of the continuous light having the weak light intensity and the optical pulse having the specified light intensity should be adjusted in order to measure strain and/or temperature with high accuracy and high spatial resolution.
Further, with the distributed optical fiber sensor according to the background art, it has been difficult to sense minute strains equal to or below 200μ∈ if strains are equally distributed in a wide range (e.g. 25-fold or more than the set spatial resolution).